Signal processing systems relying on a light emitting diode (LED), a photographic masks and an area-array charge coupled device (CCD) have been used to perform linear operations generically characterized by the equation ##EQU1##
where: g.sub.m represents the output values from the CCD horizontal shift register, h.sub.mn represents optical transmittance values of an M by N element array mask and f.sub.n represents an analog sampled-data input sequence. A typical system is disclosed in U.S. Pat. No. 3,937,942 issued to Keith Bromley et al., and entitled "Multi-Channel Optical Correlation System." An aligned mask and CCD are illuminated by an LED and electrooptically cooperate for cumulatively shifting charges in the CCD to generate a sequence of cumulative charge outputs indicative of the degree of correlation between the signal which drives the LED and the signals recorded on each column of the matrix mask. Thusly, identification of an input signal can be made by its simultaneous comparison with a large plurality of known stored reference signals. The electrooptical processor was used solely for the purpose of pattern recognition, that is, the comparing and correlating an unknown function or signal to a number of known functions so that the identity of the unknown function could be determined.
Another electrooptical processing technique is disclosed in U.S. Pat. No. 4,009,380 and is entitled "Electro-Optical System for Performing Matrix-Vector Multiplication". This patent was issued to Richard P. Bocker et al., and had a modulated LED transmitting light through a mask and onto a photo-responsive integrating detector, such as an area-array CCD. As in the previous patent, the shift rate of the charge packets within the CCD is synchronized with the modulation of the LED and develops cumulative signals in the CCD which correspond to the product of the input sequence times the signal information contained in each element of the mask to achieve a matrix-vector multiplication. This system is an adaptation of optical techniques to facilitate the making of predetermined mathematical values, called linear transformations, which hithertofore were produced by laborious mathematical procedures carried out by a series of lengthy, complex, and detailed individual mathematical computations. Additional capabilities of the patented system appeared in the July 1974 issue of Applied Optics 7, on pages 1670 et seq., in an article entitled "Matrix Multiplication Using Incoherent Optical Techniques" by Richard P. Bocker. The encoding of a matrix of analog information on a two-dimensional binary optical transparency is discussed as well as the proven feasibility of demonstrating a one-dimensional discrete finite Fourier transform.
A further disclosure regarding a combined LED, photographic mask, and area-array CCD is investigated and elaborated on at length by Michael A. Monahan et al., in the article entitled "The Use of Charge Coupled Devices in Electro-Optical Processing" appearing in the Proceedings of the 1975 International Conference on the Application of CCDs, page 217. This article concerns itself with line-array CCDs and area-array CCDs arranged in much the same manner as the two aforesighted patents to effect matrix multiplications and transformations.
Further research in the electrooptical signal processing field is disclosed in the article "Incoherent Optical Signal Processing Using Charge Coupled Devices (CCDs)" as it appears in SPIE Vol. 118, Optical Signal and Image Processing (IOCC 1977). The capabilities of the combination of an LED, photographic mask, and a CCD are shown to be increased such that a processor can perform a variety of linear transformations, multi-channel cross-correlations, filtering, and high-density read-only memory applications. Nonlinear and recursive filtering capabilities are also within the capabilities of this arrangement when real-time programmable masks are incorporated.
Three recent articles, coauthored by the present inventor, concern themselves with developments in the electrooptical signal processing field. A paper, "An Electrooptical Processor", by Michael A. Monahan et al., invited for the Proceedings of the Technical Program, Electro-Optics/Laser '78 Conference and Exposition, Boston Massachusetts, pp 479-487 on 19-21 September '78, discusses the design of optical cavities to provide uniform illumination of the CCD by the LED. Another paper by Anthony C. H. Louie et al., called "The EOP-A CCD-Based Electro-Optical Processor" appeared in Proceedings of the 1978 International Conference on the Application of Charge Coupled Devices, San Diego, California pp 3A32-3A41, 25-27 October 1978. Circuit diagrams were disclosed which showed LED-driving and CCD-clocking circuitry. A later publication by Keith Bromley et al., entitled "An Electro-Optical Signal Processing Module" appeared in the Digest of Papers, 1978 Government Microcircuit Applications Conference, Monterey, California pp 336-340, 14-16 November '78. This publication discloses a modified CCD architecture to allow increased speed and flexibility in electrooptical signal processing applications.
Thus, from the foregoing, it is apparent that electrooptical processors have evolved into a highly useful family of instrumentations for performing a variety of complex mathematical functions. However, to date, these functions have not included the compressing and expanding of audio signals within their original temporal relationship so that speech, for example, can remain unscrambled and intelligible.
One method of frequency compression and expansion is disclosed in a paper "Theory of Communication, Part 3: Frequency and Expansion" by D. Gabor in the J. I. E. E. (London) vol. 93 (LLL), pp 429-457 November 1946. The theory and mechanical devices allowed for a somewhat acceptable frequency conversion; however, the mechanical apparatus was rather complicated, bulky and of questionable long term reliability.
The article "The Digital Delay Line Revisited" by Richard Factor as it appeared on page 30 of the May 1976 issue of dB Magazine bespoke of pitch changing using random access memories (RAM). The electronic system that accomplishes frequency shifting is complicated and the use of RAM along with A/D and D/A converters necessitates complex switching sequences.